High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
It has been previously disclosed in co-pending application Ser. No. 662,910 filed Mar. 1, 1976, assigned to the same assignee as herein, that glass reinforced thermoplastic compositions of a polycarbonate resin and poly (1,4-butylene terephthalate) can be molded to articles having greater resistance to warpage in comparison with glass fiber reinforced poly(1,4-butylene terephthalate) resins. It is further disclosed in co-pending application Ser. No. 753,861 filed Dec. 23, 1976, assigned to the same assignee as herein, that zinc stearate when added to polyester polyblends elevates notched Izod impact strength, while maintaining unnotched impact strength, flexural strength and tensile strength and dramatically reduces sample-to-sample variability in elongation. Also it is disclosed in co-pending application Ser. No. 753,863, filed Dec. 23, 1976, and assigned to the same assignee as herein, that glass fibers in combination with a mineral filler provide moled articles with reduced warpage. In addition, it is disclosed in co-pending application Ser. No. 747,635, filed Dec. 6, 1976, assigned to the same assignee as herein, that compositions comprising poly(butylene terephthalate), poly(ethylene terephthalate), a polycarbonate and glass fiber have improved resistance to warpage. It is disclosed in co-pending application Ser. No. 778,945, filed Mar. 18, 1977 that blends of a poly(1,4-butylene terephthalate) resin and a polycarbonate resin reinforced with fibrous glass, when admixed with a small amount of zinc stearate possess even less inherent warpage in the moled article and good moldability when compared with compositions of glass fiber reinforced poly(1,4-butylene terephthalate).
It has now been discovered that flame-retarded, reinforced molded compositions of high molecular weight linear polyester resin, like poly(1,4-butylene terephthalate), possesses very little inherent warpage in the molded article, if a warp resisting amount of polyphenylene sulfide powder is added to the molding composition. The improved resistance to warpage is achieved without any appreciable decrease in other mechanical properties, such as notched Izod impact strength, tensile strength, modulus and flexural strength.